A Half-Century Studies on Epidemiological Features of Ancylostomiasis in China: A Review Article.

Background
Ancylostomiasis is a prevalent and global parasitic disease, including China. A systematic review is significant to understand the epidemiological features of hookworm and provide guidance for prevention and treatment.


Methods
We systematically searched academic databases and assessed 944 papers published from 1955-2015 to establish the comprehensive analysis of prevalence of hookworm disease in China. We searched Chinese databases, including CNKI, Wanfang and VIP, for literature with the subject word "Ancylostomiasis and hookworm". The data were analyzed with SPSS 19.0 software using Spearman correlation analysis. Results were statistically significant for a P-value of <0.01.


Results
The search yielded 532,151 cases from epidemiological investigation and 7294 cases based on hospital diagnosis. Hookworm infection was highest (15.83%) in Fujian province, with high rates also found in East China, Southwest China, Central China and Southern China and lower rates in Northwest China, North China and Northeast China. In terms of occupation, farmers had the highest proportion of infections (72.54%). There was no correlation between epidemiological investigations and hospital-diagnosed cases. However, there was significant positive correlation between hospital-diagnosed cases and misdiagnosed cases. The proportion of hospital-misdiagnosed cases was 32.80%.


Conclusion
Ancylostomiasis is a serious public health problem that negatively influences health and hinders socioeconomic development. Positive measures are required by both health services and individuals to prevent and control hookworm disease.


Introduction
The Ancylostomatidae worms are collectively known as hookworm and comprise two species: Ancylostoma duodenale and Necator americanus (1). Hookworm adults mainly live in the human small intestine (2). Their eggs are excreted to the environment in feces and gradually develop in vitro into filariform larvae in warm, moist loose soil with sufficient oxygen (3). Obvious thermotropism and hygrotropism are demonstrated by filariform larvae, and in soil they are stimulated by body surface temperature to strengthen their mechanical puncturing movement and enzymatic action. They are able to puncture the skin actively through hair follicles, sweat glands and skin wounds (4), and can also invade humans through perioral and transplacental routes. Of the intestinal nematodes, hookworm significantly impairs the health of infected individuals (5). The adult worms damage the intestinal mucosa causing gastrointestinal dysfunction, which can result in long-term chronic blood loss, leading to severe anemia (6). Ancylostomiasis is widely globally distributed (7), but mainly found in tropical and subtropical regions (8). In China, the vast rural areas south of the Yellow River are the main endemic regions (9). Two national surveys on parasitic diseases have been conducted in China, the first from 1988 to 1992 and the second from 2001 to 2004 (10)(11). The first survey indicated that 194 million people were infected with hookworm, which translated as 17.2% of the total population. According to the second survey, the National Survey of the Major Human Parasitic Diseases in 2005, the number of people infected with hookworm was about 39.3 million, which was 6.12% of the total population. Our study reviews hookworm infection using information published on the database over the past six decades. We collected information on the epidemiological characteristics of the disease, including infection status, risk factors, high-risk groups and misdiagnosis. An analysis was performed to characterize the epidemiology and infection trends, to guide diagnosis, prevention and control leading to the long-term aim of disease elimination.

Article selection
We searched Chinese databases, including CNKI, Wanfang and VIP, for literature with the subject word "hookworm". We also searched literature with both the subject words "China and hookworm" in the PubMed, Ovid, SpringerLink, Sci-enceDirect and Embase databases. Searched literature ranged in date from 1955 to 2015. Fulltext articles found were retrieved and selected for assessment according to inclusion and exclusion criteria.

Inclusion and exclusion criteria
Articles on hookworm infection that were epidemiological investigations and hospital case reports were included. There were no restrictions to literature type. The following exclusion criteria were applied: 1) duplicates were excluded, with the more recently published article or the one with more detailed data retained; 2) articles lacking numerical data on hookworm infection were excluded; and 3) articles reporting molecular biology studies, animal experiments, drug treatments, pathogenic mechanisms or hookworm prevention were excluded.

Factor analysis
We analyzed general infection status, occupation and regional distribution, and clinical data including provincial distribution, reason for admission, clinical characteristics, organs affected, examination methods, infection progression over 5-year periods and misdiagnosis. Cases with initially inaccurate, delayed or incomplete diagnosis causing delayed or untargeted treatment were considered to be misdiagnosed.

Statistical analysis
Excel 2007 was used to establish a database, and SPSS 19.0 software was used for statistical analysis. The results were considered statistically significant if the P-value was <0.01. The purpose of the study was to investigate the epidemiological characteristics of hookworm disease in China. Data analysis included the evaluation of epidemiological data and the analysis of hospital diagnostic data. The specific objective was to evaluate for correlation between the number of positive cases and hospital-diagnosed hookworm disease cases, in order to understand the relationship between hookworm infection and the number of patients. Spearman correlation analysis was used to evaluate the correlation between these two discrete variables (12). To evaluate the accuracy of the diagnosis of hookworm disease in hospital, the correlation between the number of hospital diagnostic cases and the number of misdiagnosed cases was eval-uated. These two variables were discrete and so Spearman correlation analysis was used. Correlation analysis using the Spearman's coefficient measures the strength and direction of the monotonic association between two discrete variables (13). A monotonic relationship is one that does either of the following: (1) as the value of one variable increases, so does the value of the other variable; or (2) as the value of one variable increases, the other variable value decreases (14).

Article evaluation
We retrieved a total of 3395 articles from the databases. Of these, 2451 did not meet the study inclusion criteria (i.e., missing data: 759 articles, animal experiments: 596, discussion of pathogenic mechanisms: 477, drug treatments: 392, reviews: 215, repeated articles: 12). A total of 944 articles were included in the final analysis ( Fig. 1). Of these, 385 articles featured hospital diagnosis of hookworm and 559 articles reported on epidemiological investigations. In addition, 110 articles were reports on misdiagnosis of hookworm infection in hospital.

Hookworm infection status according to occupation
The 944 articles conforming with the inclusion criteria totaled to 539 445 cases of hookworm infection, including 532 151 cases (98.65%) from epidemiological investigations and 7294 patients (1.35%) diagnosed at hospitals. Infected cases were divided by occupation into farmers, students, preschool children, laborers, the elderly, prisoners, soldiers, fishermen, police and occupation not detailed. Analysis showed that farmers were a high-risk group for ancylostomiasis (Table 1).

Affected organs and diagnostic method for hospital-diagnosed cases
There were definite reports on affected organs for 3847 hospital-diagnosed cases. The host's intestinal tract was the most commonly affected, mainly the duodenum, followed by multiple organs affected.
Hookworm adults parasitize the human small intestine and their eggs are excreted with the host feces; thus host feces is the material used for clinical examination (15). The common methods of analysis for hookworm infection are the direct smear method, saturated saline float method, modified Kato-Katz method, precipitation method and hookworm larva culture method (16). It is also regarded as evidence of diagnosis if worms are found by endoscopy, gastroscopy or other gastrointestinal endoscopy methods. The 532 151 cases from epidemiological investigation were all diagnosed by stool examination.
Of the hospital-diagnosed cases, 3328 (45.63%) were diagnosed by stool examination, 3314 (45.43%) by gastrointestinal endoscopy and the remaining cases by the above two methods.  Hospital-diagnosed cases displayed an increasing trend year by year, and increased at a faster rate during 1975-1980 and 2005-2010 than in other phases (Fig. 4). There was significant positive correlation between the number of epidemiological investigation cases and hospital-diagnosed cases (r=0.091, P=0.778).

Discussion
Ancylostomiasis, schistosomiasis, malaria, filariasis and leishmaniasis were listed as five major parasitic diseases in the early Peoples' Republic of China. Ancylostomiasis is widely prevalent in China (17) and is a public health problem with a considerable negative influence on health, hindering socioeconomic development (18). The pathogenic mechanism of both hookworm species is similar (19)(20), with their eggs developing into infective larvae in vitro (21), which then puncture human skin (22) to migrate and reach the small intestine (23). Hookworm eggs were express as eggs per gram of feces. (EPG). Hookworm infections were designated as light (1-399 EPG), moderate (400-3000 EPG) or heavy> 3000 EPG.
Hookworm species (A. duodenale and N. americanus) was determined by morphological identification of third-stage larvae which were successfully reared from eggs by coproculture from people. For identification of subjects with mixed or single hookworm infections, at least 100 thirdstage larvae were examined (24). Hookworm adults usually live in the upper small intestine and damage intestinal mucosa by biting with their broad teeth. Hookworm can cause Ancylostoma dermatitis when they puncture skin, as well as anemia and other digestive tract symptoms. The spread of hookworm is associated with the natural environment, farming methods, lifestyle habits and other factors (25), with filariform larvae needing suitable temperature, humidity and soil nutrition to survive in the external environ-ment (2). Thus, hookworm infection prevalence differs by region (26). According to this review, hookworm infection rates were higher in East China, Southwest China, Central China and Southern China than in Northwest China, North China or Northeast China. Infection rates were higher in Fujian (15.83%), Sichuan (14.61%) and Guangxi (9.90%) provinces and other southern areas. However, Beijing (0.004%), Tianjin (0.002%), Liaoning (0.0002%) and other northern provinces had lower hookworm infection rates.
In general, the provinces with higher hookworm infection rates have abundant rainfall, mild climates and loose fertile soil, highly suited to the growth and spread of hookworm larvae (27). Their communities are more likely to use untreated human feces to directly irrigate and fertilize crops (28), and farmers are more likely to labor in fields barefoot. Poor dietary lifestyle habits such as eating unwashed fruit and vegetables (29) contribute with these other behaviors to human contact with soil contaminated by filariform larvae and thus infection with hookworm (30). This makes farmers a high-risk group for ancylostomiasis, consistent with previous reports. The data also suggested that infants were infected from diapers and clothing polluted by hookworm larvae, as well as by sleeping in baskets and playing in fields (31). These are actions that may result in direct and indirect hookworm infection. Other infant infections may be caused by eating unwashed food or through the placenta (32 (37). Stool examination prior to gastrointestinal endoscopy may improve the diagnostic rate. Although hookworm disease can easily be confused with other diseases, medical treatment is highly effective (38)(39), with pyrantel, levamisole, mebendazole and albendazole commonly used (40). Iron and vitamin C are also recommended as supplements to pharmacotherapy (41). We conclude that the possibility of ancylostomiasis should be considered in a number of situations, including patients from rural areas with cryptogenetic gastrointestinal bleeding or longterm chronic hemorrhagic anemia (15); patients whose fecal occult blood is positive but with digestive tract cancer, ulcer and other common causes excluded by routine examination; and patients in frequent contact with soil such as farm-ers and miners (42). A careful medical history should be taken and routine stool examination applied (43). It may be necessary to screen more than once, combining the saturated saline float method, modified Kato-Katz method and gastrointestinal endoscopy, to improve detection rates (44) and decrease missed diagnosis and misdiagnosis rates.
Our results showed that incidence was highest between 1990 and 1995, and incidence determined by hospital diagnosis increased over the 5year periods. We summarize the reasons for these results. First, living conditions have improved rapidly with greater socioeconomic development since the Reform and Open, which caused a decrease in hookworm infection rates at that time. Second, knowledge of and record-keeping on clinical cases of ancylostomiasis has improved (45); literature about ancylostomiasis was unusual before 1990, and it is likely that more published research on the disease will have improved knowledge (46). Third, health promotion has been strengthened in recent years (47). Fewer people work barefoot in fields and better hygiene and lifestyle habits have gradually developed. Moreover, health services have attached greater importance to the prevention and control of hookworm disease (48). Manure management should be standardized to reduce environmental pollution and the opportunity for hookworm eggs to infect soil (49). Finally, the high rate of hookworm infection in the 1990-1995 period may be related to floods during this time. Further investigation would assist in confirming these hypotheses.

Conclusion
Prevention and control of hookworm disease should focus on strengthening publicity and encouraging the participation of communities. Harmless fecal fermentation methods, improvement of water supply and sanitation, environment beautification and new energy production methods will all reduce soil contamination from hookworm eggs. Personal protective measures also need strengthening, such as promotion of the use of protective clothing and boots to farmers, to avoid exposed skin contact with soil especially after rain. In summary, a focus on improved personal, dietary and environmental hygiene will assist in reducing the prevalence of ancylostomiasis.

Ethical considerations
Ethical issues (Including plagiarism, informed consent, misconduct, data fabrication and/or falsification, double publication and/or submission, redundancy, etc.) have been completely observed by the authors.